Our crystals of SV40 virus are cubes and can be grown to about 0.2mm on edge. They belong to the space group I23 and have a unit cell dimension of about 560 A. They appear to be isomorphous to the crystals of polyoma virus being studied by our collaborators and ourselves. The crystals diffract to about 30 A resolution so that only a very low resolution structure is possible. Data collection for this has begun. We have been able to grow sizeable, two-dimensional arrays of SV40 particles for study in the electron microscope. These also diffract very badly with a resolution limit of about 60 A. Reconstructions based on optical filtration, however, do not indicate rotational disordering so that improvement here is possible.